A need exists in industry for non-destructive inspection (NDI) of composite structures assembled with adhesive bonds. Among other techniques, laser bond inspection (LBI) has proven useful. Normally, LBI involves depositing laser energy onto the front surface of a bonded article and generating compression waves that reflect off of the back surface of the bonded article as tensile waves. The tensile waves predominantly provide the stresses that interrogate the bond. In the simplest form of LBI, the laser energy is deposited uniformly onto a circular area on a bonded article's exposed surface. The energy deposition results in a series of alternating compression and tension waves that propagate into the bonded article. These waves reflect from the back surface, or other impedance discontinuities, of the bonded article, and interact with one another to produce regions of varying tensile stress. If a tensile wave of sufficient strength encounters a weak bond, the bond will fail (or disbond). The variation in peak tensile stress with depth in the bonded article depends, in part, on the ratio of the beam diameter to the thickness of the bonded article.
The disbonded interface may be detected by ultrasound after the laser exposure, or by techniques used simultaneously with laser exposure, such as a velocity interferometer for surfaces of any reflectance (VISAR), an electromagnetic acoustic transducer (EMAT), capacitance probes, or piezoelectric ultrasonic transducers (UT).
If a bond in a thick bonded article (e.g., >¼ inch) is near the front surface where the laser energy is applied, high fluences may be required for interrogation because the interrogating tensile wave must have sufficient amplitude to stress the bond adequately after the initial compressive and reflected tensile waves have traveled nearly twice the thickness of the bonded article. Material failure may occur at the back surface (spalling) at fluences that are still inadequate (due to stress wave attenuation) to properly stress the bond near the front surface.
A similar problem occurs when attempting to interrogate a particular bond near the front surface of a bonded article having two or more bonds. The fluences required to stress the bond of interest near the front surface may undesirably damage good laminate bond(s) nearer the back surface.